Mitral regurgitation (MR) is a frequent complication of myocardial infarction (MI) and left ventricular (LV) dysfunction that doubles mortality, but the survival benefit of standard annular ring reduction therapy is intensely debated. LV remodeling often progresses after ring reduction, causing recurrent MR. We must therefore resolve in a controlled fashion whether MR contributes to remodeling; whether the benefit of repair is limited by being typically late; and why aggressive remodeling continues post-repair. In the initial grant period, we developed an apical MI model (no intrinsic MR) with standardized MR-type flow through an LV-to-LA shunt. In that sheep model, moderate MR flow typical of post-MI patients strongly increased LV remodeling at the whole-heart, cellular and molecular levels versus MI or MR alone, with a biphasic pattern of failed attempts at compensation (rise, then exhaustion of hypertrophic, anti-apoptotic signals). Early MR repair by shunt closure reversed remodeling, while early transgenic overexpression of sarcoplasmic reticulum Ca+2-ATPase (SERCA2a) improved it. Building on that, the current proposal addresses the central hypothesis that exacerbation of post-MI LV remodeling by MR can be reduced by appropriately timed molecular or mechanical interventions, with the following specific aims: 1) To test the hypothesis that the reversal of remodeling by MR repair decreases over time, in parallel with a reduced molecular momentum for reverse remodeling, emphasizing the benefit of early repair. 2) To test the hypothesis that remodeling can be reduced by transgenically modulating the stress-sensitive calcium cycling pathway that is exhausted in the decompensated phase of MI+MR. In a clinically relevant spectrum of anterior and inferior MIs with MR, this molecular intervention will be tested for its ability to improve reversal of remodeling in response to delayed MR repair. This would correspond to improved postoperative function, extending the window for reverse remodeling beyond any point of no return due to delayed repair. The resubmission is focused on the interaction between delayed repair and molecular interventions, supported by quantitative data from SERCA2a gene transfer studies. The research team combines expertise in surgical modeling (Dr. Gus Vlahakes), quantitative 3D cardiac imaging, and the molecular biology and transgenic modification of LV remodeling (Drs. Roger Hajjar and Ronen Beeri). These aims focus on the current questions regarding the central impact of MR repair on LV function to increase our mechanistic understanding and identify potential therapeutic targets.